Medical science has an increasing need for quick, accurate determination of analytes in blood or other body fluids. Traditionally, assays for analytes have been performed by laboratories and required skilled technicians, complex apparatus and reagents, and considerable time in order to obtain accurate results. A great number of qualitative and some quantitative devices and methods have been developed which eliminate or decrease the need for laboratory diagnostic services. Many of these devices and methods include test strips or dip sticks which can be exposed to blood or another body fluid in order to obtain a diagnostic result. A common example of this technology includes the various test products for determining glucose concentration for diabetics to monitor their glucose levels.
Flow devices for quantitative analytical measurements are currently available. However, their utility has been significantly hampered by materials and processes for producing the devices which are unsuitable for use with fully automated and internally controlled manufacturing. The need for such automation and process control is particularly important for the reliability of devices which do not rely on comparative measurements involving the use of single or multiple calibration standards prior to running the test sample in the final analysis.
Among the problems encountered in manufacturing these flow devices are the problems of trapping the indicator in such a manner that a reliable, accurate determination can be made. A number of prior art workers have described such assay devices, but heretofore there has not been a method that can be automated for making a reliable quantitative device.
Previous methods for preparing flow devices for quantitative measurements involve immobilizing an indicator dye on an absorbant support, typically a special paper in roll or sheet form, where the immobilization process involves chemical reactions which are difficult to control, and washing steps. It is very difficult to achieve uniform distribution of immobilized dye molecules throughout the sheets or from beginning to end of a roll of this paper. Additionally, the mechanical effects of processing can degrade the physical integrity of a paper and result in variable wicking and volume of fluid uptake. The variations in the chemical and physical behavior of sheets prepared in this manner are significant, and adversely affect the precision of assays conducted with these devices.
Przybylowicz, in U.S. Pat. No. 3,993,158, discloses an integral element for analyses of liquids which comprises an isotropically porous spreading layer which contains microparticles trapped in a thin film of hydrophilic polymer such as polyvinyl alcohol or gelatin. The only purpose of this layer is to spread liquid quickly so that even flow of liquid occurs into and through the layer to the bottom thereof, where another layer takes up the liquid to subject its components to a specific analytical test. The particles in the spreading layer do not contain any covalently linked chromatic indicators.
Pearce et al., in U.S. Pat. No. 4,258,001, disclose another type of spreading layer containing polymeric, nonporous particles linked with small amounts of adhesive, which provide large open spaces between the beads because the adhesive only connects individual beads without filling the interstitial spaces. This makes for a very effective spreading layer. Any chemical reactants in the layer are adsorbed to the beads and are not covalently bound. Such a layer would not be suitable for use in a flow-through device because reagents such as the dye chromogen would leach out of the spreading layer if subjected to lateral flow of the sample.
Vogel, in U.S. Pat. No. 4,312,834, uses particles in films to make the film porous to larger molecules. Immunoglobulins and albumin are important components of human blood which do not easily diffuse into polymer films containing chemical reactants. When these films are "opened" by the addition of particles such as fumed silica or microcrystalline cellulose, these large molecules are given access to chemical integrated into the film. The particles trapped in the films described by Vogel, however, are not covalently bound to the chemical reactants, and are therefore not suitable for flow devices in which lateral flow may elute reagents and reaction products. This elution precludes the formation of a distinct, sharp front of a color bar needed to derive quantitative visual results.
Pierce et al., in U.S. Pat. No. 4,258,001, disclose an element for the analysis or transport of liquids comprising a plurality of heat-stable, organo-polymeric particles non-swellable in and impermeable to the liquid, and an adhesive concentrated at the particle surface areas contiguous to adjacent particles bonding the particles into a coherent, three-dimensional lattice that is non-swellable in the liquid.
Siegel et al., in European patent No. 345 460 disclose a test device using covalently immobilized colored dyes wherein a second component of a two component dye system is covalently bonded to a matrix. Color is formed when a first component, which may be the analyte or another dye component, covalently couples to the second component. The color formed is covalently immobilized to the matrix.